Vol. 57, Issue 3, 564-567, March 2000

Calmodulin Increases the Sensitivity of Type 3 Inositol-1,4,5-trisphosphate Receptors to Ca²⁺ Inhibition in Human Bronchial Mucosal Cells

Ludwig Missiaen, Humbert DeSmedt, Geert Bultynck, Sara Vanlingen, Patrick Desmet, Geert Callewaert, and Jan B. Parys

Laboratorium voor Fysiologie, K.U.Leuven Campus Gasthuisberg O/N, Leuven, Belgium

Inositol-1,4,5-trisphosphate (IP₃) releases Ca²⁺ from intracellular stores by binding to its receptor (IP₃R), a multigene family of Ca²⁺-release channels consisting of IP₃R1, IP₃R2, and IP₃R3. IP₃R1 is stimulated by low cytoplasmic Ca²⁺ concentrations and inhibited by high concentrations. Discrepant reports appeared about the effect of cytoplasmic Ca²⁺ on IP₃R3, showing either a bell-shaped dependence or only a stimulatory phase with no negative feedback by high Ca²⁺ concentrations. We investigated how calmodulin interfered with the feedback of cytosolic Ca²⁺ on the unidirectional IP₃-induced Ca²⁺ release in permeabilized 16HBE14o- bronchial mucosal cells, where IP₃R3 represents 93% of the receptors at the mRNA level and 81% at the protein level. Calmodulin inhibited the Ca²⁺ release induced by 1.5 µM IP₃ with an IC₅₀ value of 9 µM. This inhibition was absolutely dependent on the presence of cytosolic Ca²⁺. Ca²⁺ inhibited the IP₃R with an IC₅₀ value of 0.92 µM Ca²⁺ in the absence of calmodulin and with an IC₅₀ value of 0.15 µM Ca²⁺ in its presence. It is concluded that: 1) IP₃R3 can be inhibited by calmodulin, 2) IP₃R3 is inhibited by high Ca²⁺ concentrations, and 3) calmodulin shifts the inhibitory part of the Ca²⁺-response curve toward lower Ca²⁺ concentrations.